This invention relates to the treatment of aluminum electrolytic capacitor foil in which the foil is contacted, after etching but prior to anodizing, with a phosphate solution and then heat treated for 30-90 seconds to form a thermal oxide layer of controlled thickness. On subsequent anodization, the presence of the thermal oxide layer modifies the barrier oxide layer increasing its dielectric strength and thus the capacitance.
Thermal oxidation of aluminum foil is known to increase capacitance but also leads to instability and high leakage currents. The length of the heat treatment is critical as long times produce a poor product.
It has also been known to anneal aluminum foil prior to anodization. This heat treating is carried out at high enough temperatures and long enough times to permit at least partial recrystallization or growth in metal grain size decreasing the amount of amorphous metal or providing a better temper.
Other processes are well-known in which the foil is cleaned with acid and then heated. Generally, nitric acid is used as nitrate residues are relatively easy to remove from aluminum foil surfaces by heating the foil.
Phosphate and phosphoric acid treatment of aluminum foil to inhibit hydration of the foil is also well-known. This treatment is sometimes followed by a drying step at relatively low to moderate temperatures.